1. Field of the Invention
The present invention relates a piezoelectric type actuator, and more particularly to a piezoelectric type actuator having a stable resonance frequency.
2. Description of the Related Art
A conventional diaphragm type micropump which uses a piezoelectric type actuator is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 5-296150). In the conventional example of the piezoelectric type actuator, a ceramic plate on which a piezoelectric vibration element is installed is used as a diaphragm. The diaphragm is installed on the wall section of a housing of a piezoelectric type actuator such that the diaphragm can vibrate in forward and back directions. An inlet port and outlet port are provided for a room in the front of the diaphragm and the room of the back of the diaphragm acts as a pump room. The diaphragm vibrates in the forward and back directions in response to drive vibration from a driving circuit so that a pump operation is carried out.
However, in this conventional example, a problem of an area ratio between the inlet port and the outlet port and another problem of the position of the inlet port are only considered. The easiness of assembly of the piezoelectric type actuator and the relation of the piezoelectric type actuator and a driving circuit for driving it are not considered at all.
FIG. 1 shows a cross sectional view of a piezoelectric type pump after assembly to which another conventional piezoelectric type actuator using a piezoelectric vibration element is applied. Referring to FIG. 1, the piezoelectric type pump is composed of an upper member 102, a piezoelectric vibration element 104 and a lower member 106. The piezoelectric vibration element 104 is held between the upper member 102 and the lower member 106 in a given pressure. FIG. 2 is a partially exploded cross sectional view of a holding section in which the piezoelectric vibration element 104 is held between the upper member 102 and the lower member 106. FIG. 3 is a cross sectional view of the dissolved piezoelectric type pump which is shown in FIG. 1.
Referring to FIGS. 1 to 3, the upper member 102 has an upper plate section 140 and a circular cylindrical side wall section 142 which extends downward from the edge portion of the upper plate section 140. An outlet port 112 is formed in the upper plate section 140. In the side wall section 142, an inlet port 114 and a hole 115 for introducing a lead wire to the piezoelectric vibration element 104 are formed. The outer side portion 116 of the side wall section 142 of the upper member 12 extends downward longer than the inner side portion 118 thereof. Thus, a step is formed between the outer side portion 116 and the inner side portion 118 in the side wall section 142. In the surface of the inner side portion 118, a projection portion 118a is formed. In the outer side portion 116, holes 151-1 and 151-2 are formed for fastening.
In the piezoelectric vibration element 104, a piezoelectric element 124 composed of PZT (PbTiO.sub.3, PbZrO.sub.3) is installed on a vibration plate 122. The piezoelectric element 124 is attached on the vibration plate 122 with adhesive material. A lead wire 128 is connected to the piezoelectric element 124 by solder 126. The lead wire 128 covered by insulator is connected to a driving circuit (not illustrated) through the hole 115 which is provided in the side wall section 142 of the upper member 102. The ground line of the driving circuit is connected to the lower member 106.
A drive signal which has a predetermined frequency is supplied from the driving circuit to the piezoelectric element 124 via the lead wire 128. When the drive signal is applied to the piezoelectric element 124 via the lead wire 128, the piezoelectric element 124 vibrates so that the vibration plate 122 vibrates according to the vibration of the piezoelectric element 124. In this way, a pumping operation is accomplished.
The lower member 106 has a base plate section 144 and a circular cylindrical side wall section 134 which extends upward from the edge portion of the base plate section 144. An outer side portion 132 of the base plate section 144 which is located outside of the side wall section 134 is combined with the outer side portion 116 of the upper member 102. The side wall section 134 is combined with the inner side portion 118 of the upper member 102. In the lower member 106, screw holes 152-1 and 152-2 are formed outside of the side wall section 134 in correspondence to the screw holes 151-1 and 151-2 of the upper member 102.
The point that the efficiency becomes the best when the piezoelectric type pump is driven, i.e., the point that a maximum gas stream is accomplished is in the resonance point of the piezoelectric vibration element 104. Therefore, the driving circuit for the pump can be simplified if the resonance frequency does not change due to pressure, temperature and so on, that is, if the this resonance point does not change due to them.
For this purpose, in the piezoelectric type pump for the gas rate microsensor, the upper member 102 and the lower member 106 are assembled in the following manner, such that the piezoelectric vibration element 104 is held between the upper member 102 and the lower member 106, as described above.
First, the piezoelectric vibration element 104 is positioned on the side wall section 134 of the lower member 106. Next, the upper member 102 and the lower member 106 are engaged with each other such that the outer side portion 116 of the upper member 102 is combined with the outer side portion 132 of the lower member 106, and such that the inner side portion 118 of the upper member 102 is combined with the side wall section 134 of the lower member 106. In this case, the piezoelectric vibration element 104 is held between the project portion 118a of the inner side portion 118 of the upper member 102 and the surface of the side wall section 134 of the lower member 106, as shown in FIG. 2. After that, the upper member 102 and the lower member 106 are fastened with screws using the screwing holes 151-1, 151-2, 152-1 and 152-2.
As mentioned above, in the structure of the conventional piezoelectric type pump, the peripheral portion of the vibration plate 104 on which the piezoelectric element 124 is mounted is held between the upper member 102 and the lower member 106 with a holding pressure.
The piezoelectric vibration element 104 is distorted because of the stress, when stress is applied to a part of the piezoelectric vibration element 104. The distortion influences the frequency characteristic of the piezoelectric vibration element 104. That is, the load on the holding portion between the upper member 102 and the lower member 106 changes a resonance frequency of the piezoelectric vibration element 104.
As described above, in order to simplify a driving circuit, it is desirable that the resonance frequency does not change because of the conditions such as sealing pressure, temperature and so on. However, there is a problem in that the frequency characteristic of the piezoelectric vibration element changes because of the fastening torque of the screws. Also, it is made apparent that the temperature characteristic of the resonance frequency depended on this screw fastening torque. This is because the frequency characteristic is affected by the magnitude of stress which is generated in the holding portion of the piezoelectric vibration element.
For these reasons, the above-mentioned conventional piezoelectric type pump is assembled in the following manner. That is, each of 4 screws is fastened while the torque is managed in the state in which the upper member 102 and the lower member 106 are pushed to each other with a holding pressure for holding the piezoelectric vibration element 104 incorporated between them. As a result, a desired frequency characteristic can be obtained. In this assembling method, however, there is a problem in that it takes a long time for assembling one pump. In this way, the productivity is low since the precise management of screw fastening torque must be carried out to accomplish the desired frequency characteristic in the piezoelectric type pump having the conventional structure.
FIG. 4 shows an example when a piezoelectric type micropump is applied to a circulation type closed flowing path gas rate sensor in which the slant state of a gas stream generated when an angular speed acts on the sensor is electrically detected.
The gas is spouted out from an outlet port 207 of a diaphragm type piezoelectric micropump (204, 203, 209) by driving the micropump and flows through a flow path 211 which is formed in a casing 210 of the sensor. Then, the gas is spouted out for the inside of the sensor from a nozzle hole 212. The gas which is spouted out for the inside of the sensor causes a gas stream which moves for a pair of heat wires 241 and 242 which are provided in the flow path. When a movement of an angular speed is applied to the sensor, the gas stream flowing through the inner gas flow path is deflected. A sensor signal is outputted to correspond to the difference between the thermal outputs which are generated in the heat wires 241 and 242 by the deflected gas stream. In the above circulation type closed flowing route gas rate sensor, the gas flow route is composed of the outlet port 207, the flowing route 211, the nozzle hole 212, and the inner gas lowing route 213. The load conductance in the nozzle hole 212 where the maximum resistance is provided in the whole of gas flowing route is as large as 106 to 107 (cm.sup.3 /S). In this case, a sufficient flow rate is accomplished by a limited pump ability of this micropump.
When a piezoelectric type micropump is applied to the circulation type closed flowing route gas rate sensor which is sealed with a predetermined pressure, the resonance frequency of the piezoelectric vibration element 203 changes because of the sealing gas pressure and the peripheral temperature, even if the holding pressure of the holding portion between the upper member 204 and the lower member 209 is supposed to be controlled for simplifying a driving circuit.
FIG. 5 is a measurement result indicating dependency of the resonance frequency upon the sealing pressure. As seen from FIG. 5, the resonance frequency of the piezoelectric vibration element is affected by the peripheral temperature, the holding pressure and the sealing pressure. That is, the resonance frequency of the piezoelectric type actuator changes when the sealing pressure changes even if the piezoelectric type actuator has the resonance frequency characteristic in which the resonance frequency does not almost change because of the temperature change under the atmosphere pressure. For this reason, it is necessary to correct the frequency of the drive vibration which is supplied from the driving circuit to the piezoelectric vibration element. Therefore, there is a problem in that the circuit scale of the driving circuit becomes large.